1. Field of the Invention
The invention concerns a method for producing stabilized zirconium oxide powder with a particle size of at least 100 um, preferably between 100 um and 1000 um, preferably intended for high-density ceramic materials.
2. Description of the Prior Art
Trends in ceramics development have recently been moving toward increasingly high-quality ceramic materials, with the goal of utilizing the advantageous characteristics of these materials--including good temperature resistance, great hardness and wear resistance, and good resistance to chemicals--for engineering purposes.
The first priorities in this effort are to achieve the highest possible density and strength in the shaped elements, to lower the sintering temperature, and in many applications also to increase substantially the purity of the ceramic materials.
To achieve these goals, homogeneous, easily pressed powders of the greatest possible purity, with high sinterability, must be available on an industrial scale; and it must be possible to produce them with simple methods and as cost-effectively as possible.
One promising material that can meet these requirements is partially stabilized zirconium oxide, called PSZ. In this material the tetragonal crystal structure that, in pure zirconium oxide, is stable only at temperatures above 1000.degree. C. is stabilized by the addition of suitable additives. Metal oxides such as CaO, MgO, Y.sub.2 O.sub.3, CeO.sub.2, etc. are used as stabilizing additives. An important factor here is the homogeneity of the powder mixture, which with the usual dry synthesis methods can be achieved only with difficulty and by means of laborious procedures involving several stages, such as generating precursor products by mixing the starting powder, pressing, heat treating, and lastly grinding to a fine powder mixture. Variants aimed at improving homogeneity, such as melting eutectic mixtures, have also been disclosed.
"Wet synthesis" methods have been developed to improve the homogeneity and sinterability of powder mixtures. These involve either precipitating or otherwise consolidating salt solutions which contain the components that form the ceramic, or consolidating hydrosols of these substances by forming gels. Recently, alcoholates of the metals forming the ceramic have also been used to form aels.
In coprecipitation methods, finely dispersed powders are generated--in most cases from chlorides or nitrates of the metals--by hydroxide precipitation using alkaline substances (usually ammonia), or by precipitation of easily decomposed salts of organic acids (such as oxalic acid).
DE-A 34 08 096 describes a method in which a starting powder for a CeO.sub.2 -stabilized sintered zirconium product is produced by precipitating zirconium and cerium hydroxide with aqueous ammonia. Experience shows, however, that hydroxide precipitations of this kind produce voluminous precipitates that often have poor filtration characteristics and are difficult to wash; in addition they are difficult to handle, and after drying and calcining yield large blocks that must first be laboriously sieved to produce a fine, pourable powder.
Particular attention has been paid recently to sol-gel methods, since they produce powders with high sintering activity and large surface areas. In the simplest case, sols can be produced by hydrolysis of suitable salt solutions at boiling heat (DE-A 34 08 096). When the sol is dried and then calcined, however, the result is once again coarse blocks of gel that take a great deal of extra time to grind into powder.
Conventional methods of sol-gel technology in general and for the production of Y.sub.2 O.sub.3 -stabilized zirconium oxide in particular are-described by J. L. Woodhead and D. L. Segal in "Sol-gel processing," Chemistry in Britain, April 1984, pp. 310-313. Considerable space therein is devoted to sol production, and the authors explicitly note that production costs must be commensurate with the value of the ceramic product that is generated. In reality, the procedures described in this article are extremely costly because they are chemically complex. Although solvent extraction as a method for producing stable hydrosols (which must have a large anion deficit) is relatively simple to implement with long-chain amines (R.sub.3 C--NH.sub.2, where C.sub.18 &lt;R&lt;C.sub.22), regenerating these amines demands considerable chemical effort and produces large amounts of waste. Hydroxide precipitation and subsequent peptization of the precipitate, washed to remove salts, with a little mineral acid (usually nitric acid) also yields large volumes of filtrate waste. Moreover the sols, which occur initially in dilute form, must be reconcentrated by boiling. The method involving thermal decomposition of metal salts of volatile acids, such as chlorides or nitrates, also has disadvantages: not only because of the corrosive effect of HCl or NO.sub.x vapors, but also because not all elements form such salts. In addition, peptizable oxide hydrates are obtained only under precisely defined conditions, as described using the example of thorium nitrate.
GB-B 1 181 794 describes the production of these kinds of zirconium hydrosols or hydrogels with a substantial anion deficit. It is important in this connection that a precise molar NO.sub.3 --/Zr ratio of between 1.0:1 and 1.1:1 be maintained. A major disadvantage is the fact that this sol production procedure must be preceded by the aforementioned hydroxide precipitation step, with the nitrate subsequently washed out, before peptization to the sol can occur with a specific small quantity of nitric acid. The gel produced by drying has the disadvantage that it occurs in fragments and contains a relatively large amount of nitrate. When calcined, it therefore gives off NO.sub.x and has a severely corrosive effect, and in addition must still be finely ground. Precipitation py amine extraction has the disadvantage that the amine must be regenerated and the organic solvent recovered.
The disadvantage of the variants of the sol-gel method thus consists primarily in the difficult process engineering associated with sol production, whether by precipitation of the metal hydroxides and peptization, or by removal of the anions using long-chain organic amines (solvent extraction), or by thermal decomposition of salts of volatile acids (nitric acid, oxalic acid) followed by peptization of the metal oxides.
EP 0 261 590 A2 indicates a method for producing metal oxides or mixed metal oxides, in which an aqueous solution of a zirconium salt, which can contain an additive, is atomized ultrasonically. This generates droplet sizes between 10 and 50 um.
Although the use of ultrasound has the advantage that small droplets can be generated, a disadvantage is that droplets with highly variable diameters are produced. Twin and triplet droplets are also observed. Moreover, a uniform spherical geometry is not obtained.
A method for producing a zirconium dioxide powder is described in DE 39 32 781 A1. With this, zirconium chloride containing water of crystallization is melted, and gaseous ammonia is then introduced into the melt.
A known method for producing ceramic powder is to introduce a reaction gas into an emulsion (EP 0 304 243 A1). Further methods for producing zirconium powders or other powders are indicated by U.S. Pat. Nos. 3,384,687, 4,664,894, and 4,140,771.